The present invention relates generally to photography and more specifically to a photography using a flash or strobe light.
Cameras use a variety of techniques for achieving the proper exposure of photographs. Commonly, a camera makes one or more trial photographs of a scene using selected camera settings. These trial photographs are analyzed to determine the amount of light being captured from the scene and how the exposure should be adjusted to improve the exposure level. The camera may then adjust any of several settings before taking a final photograph so that proper exposure is achieved.
Some of the camera settings which may be adjusted include the shutter time (either mechanical or electronic), the lens aperture ratio, the electronic amplification of signals within the camera (sometimes called system gain), whether a strobe light should be fired to supplement the ambient lighting of the scene, and if so, how much energy to supply to the strobe. In some cases the user of the camera may specify some or all of the settings and require the camera to adjust any remaining settings to achieve proper exposure.
In order to determine which adjustments should be made, the camera assumes a definition of proper exposure and incorporates knowledge of how each potential adjustment will affect the exposure in the final photograph.
In the case where a strobe light is used, a method is needed to determine the proper amount of energy to supply to the strobe. The more energy supplied to the strobe, the more light the strobe emits, and the brighter the exposure of the resulting photograph.
For the purposes of this disclosure, a photograph may be a numerical representation of a scene captured by a camera, and need not be a printed representation of the scene.
A common technique for determining the proper strobe energy is to take a trial photograph with the strobe energy set to a known amount. The resulting photograph is examined and its exposure quality evaluated. If adjustment is required, a new value for the strobe energy is determined. The final photograph is then taken using the computed energy value. When some areas of the trial photograph are found to be fully exposed, the strobe energy used for the final photograph may be reduced from the energy level used for the trial photograph. One indication that an area is fully exposed is that the pixel data in that area of the photograph are saturated. To be saturated means that the digital value representing those pixels is the highest value the camera can indicate, and that additional light in those pixels will not produce a higher digital value.
However, this method often fails to produce a good result when the scene contains a small area of very bright light such as a glare reflection or a self-luminous light source such as a light fixture or lamp. The camera may assume that the very bright areas were the result of the strobe energy used in the trial photograph and underestimate the amount of strobe energy required to properly expose other parts of the scene. Alternatively, the camera may interpret the bright areas as sufficient illumination for the entire scene and choose not to use the additional strobe at all, thereby causing the photograph to be underexposed. In either case, the photograph is not exposed properly.
What is needed is a technique for setting the strobe energy properly when the scene being photographed contains small very bright areas.
Strobe energy in a camera is adjusted to improve exposure. A trial exposure of the scene using only ambient light is taken. Pixels which are found to be fully exposed, or saturated, in the ambient-only trial photograph are discounted in the computation of the strobe energy appropriate for a final photograph.